1. Technical Field
The present invention relates to liquid ejecting apparatuses such as facsimile machines and printers.
2. Related Art
As an example of liquid ejecting apparatuses, an exemplary ink jet printer will be described below. An ink jet printer is provided with an inkjet recording head and a support member (also referred to as a platen) which is arranged so as to oppose the ink jet recording head. The ink jet printer is configured such that when a recording sheet is supported by the support member, a distance is defined between the ink jet recording head and the recording sheet.
In recent years, development of ink jet printers has been carried out with the aim of further improving the quality of recording by minimizing the size of each ink droplet, which is minimized to for example several picoliters. As a result, when those ink droplets are ejected from the ink jet recording head toward the recording sheet, some of them may not land on the recording sheet because the mass of each ink droplet is extremely small. They may remain suspended as a mist, thereby leading to various problems. Further, in so-called borderless recording in which recording is performed without leaving margins on all four sides of a recording sheet, ink droplets are also ejected toward an area outside the edges of the recording sheet, which makes the above described phenomenon of mist suspension more apparent.
Under these circumstances, patent documents such as JP-A-2007-118321 and JP-A-2007-118318 disclose a technique in which a potential difference is applied across an ink jet recording head, a recording sheet and a support member, and thus an electric field is generated, allowing the Coulomb force to act on ink droplets so that the ink droplets are attracted to the recording sheet.
1. Problems Associated with the High Speed Transportation of Recording Sheets
Recently, ink jet printers have been proposed which are capable of recording with extremely high throughput by employing stationary ink jet recording heads, also referred to as line heads, which do not require performance of a scanning (moving) operation. In such ink jet printers, recording sheets are transported within the apparatus through a sheet transportation path at an extremely high speed.
However, such high speed transportation of recording sheets has been found to cause some problems such as those described below. That is, if no control of the electric potential is performed on the recording sheet, the support member (platen), and the ink jet recording head (hereinafter collectively referred to as “recording unit components”), an electric field generated across the recording unit components may cause a problem such that paper dust, which has been created during cutting and attached on the edges of recording sheets, flying toward the ink jet recording head and adhering thereto. In particular, when recording sheets are transported at a high speed, greater vibration or impact will occur during the transportation, thereby causing a more significant amount of flying paper dust.
Furthermore, through friction between the recording sheets accommodated in a sheet cassette or sliding contact or contact between components in the transportation path (e.g., an edge guide or a transportation roller) and the recording sheets, frictional charging or separation charging is increased, thereby causing the recording unit components to be more strongly charged. As a result, the electric field generated across the recording unit components becomes strong, and thus electrostatic charge on paper dust per se also becomes strong, thereby Coulomb force exerting on paper dust will be increased. This may result in more pronounced paper dust adhesion to the ink jet recording head.
In addition, even when paper dust per se is not charged, the flying paper dust in the electric field is subjected to dielectric polarization or electrostatic induction which causes imbalance of electric charges in paper dust, resulting in paper dust being attracted toward the ink jet recording head.
FIG. 4 is an explanatory view showing the above problem, in which reference numerals 160 and 160a denote an ink jet recording head and a nozzle plate, respectively. Reference numeral 170 denotes a support member (a platen) and reference numeral 170a denotes each rib formed on the support member 170. Further, reference symbol P denotes a recording sheet, reference symbol Pe denotes an edge of the recording sheet P, and reference symbol d denotes a piece of paper dust. Circled plus and minus signs indicate the respective charging polarities.
The recording sheet P is neutralized, for example by means of a neutralization brush, and therefore the paper dust d attached on the recording sheet P is not charged. However, as illustrated in the enlarged view of the paper dust d, when the nozzle plate 160a is positively charged and the support member 170 is negatively charged as an example, negative charges in the paper dust d move toward a surface closer to the nozzle plate and the positive charges in the paper dust d move toward another surface closer to the support member, due to dielectric polarization (when the paper dust d is dielectric in nature) or electrostatic induction (when the paper dust d is conductive in nature). As a result, the paper dust d may be attracted toward either the nozzle plate 160a or the support member 170.
When the paper dust d adheres to the inkjet recording head, the paper dust directly obstructs the nozzle openings or, alternatively, moves onto nozzle openings during cleaning (wiping) of the nozzle surface, which may lead to missing dots.
In addition to the above described case in which the paper dust d physically obstructs the nozzle openings, filler which constitutes the paper dust d, such as calcium carbonate, may react with water in the ink and become thicker, thereby disturbing the meniscus vibration at the nozzle openings and interfering with ejection of ink droplets. Therefore, it is quite important to avoid paper dust adhesion to the ink jet recording head in order to achieve appropriate quality of recording in the ink jet printer.
2. Problems in the Related Art
As described above, JP-A-2007-118321 and JP-A-2007-118318 disclose a technique in which a potential difference is applied across the ink jet recording head, the recording sheet and the support member (recording unit components), and thus an electric field is generated, causing the Coulomb force to act on ink droplets so that the ink droplets are attracted to the recording sheet. Accordingly, if paper dust were to be simply considered in the same way as ink droplets, it is thought that it would be possible to prevent paper dust adhesion to the ink jet recording head by controlling an electric field so as to attract paper dust toward the recording sheet.
However, filler and cellulose fibers which constitute paper dust can be either positively or negatively charged according to the triboelectric series. If an electric field in a certain direction is applied across the recording unit components, in an attempt to prevent paper dust from flying toward the ink jet recording head, paper dust will be charged to the opposite polarity, resulting in failure to prevent them from flying toward the ink jet recording head.
JP-A-2003-165230, which addresses the prevention of paper dust, dust particles or the like from adhering to the periphery of a nozzle unit of an ink jet recording head, as one of the advantages of the invention, discloses a recording apparatus which includes an air duct provided around the nozzle plate and is configured to blow humidified air from the air duct during recording and recording stand-by period. However, with such a configuration, the device would be larger in size and of higher cost due to complexity of the configuration. In addition, the air blown from the air duct may undesirably cause paper dust to adhere to the recording head.
Further, JP-A-2008-213255 discloses a technique for collecting paper dust by means of a charged paper dust collecting member. However, this technique has some problems in that the paper dust collecting member does not always effectively collect paper dust due to the above mentioned opposite polarity, and paper dust accumulated on the paper dust collecting member needs to be disposed of (removed). In particular, when a significant amount of paper dust is accumulated, a slight vibration or impact may cause paper dust to be released into the air, which makes it difficult to maintain the performance over a long period of time.